Field of the Invention
This invention relates to USB (Universal Serial Bus) transmission method, and in particular, it relates to a method of extending the transmission range between USB host and USB devices for isochronous transfer.
Description of Related Art
Data transmission between electronic devices using the USB protocol has a limited range, typically a few meters. Extender devices that can extend the range of USB data transmission have been described. For example, U.S. Pat. No. 6,381,666 describes “a method and apparatus to be used to extend the range of a standard USB devices. An extended range hub is provided which comprises a Local Expander (LEX) and a Remote Expander (REX) which can be separated by up to, for example 100 meters.” (The U.S. Pat. No. 6,381,666, Abstract.) This patent explains that the main reason for the limited transmission range is the requirement that “when the Host Controller instructs a particular device to place its information onto the shared bus, the requested information must be received by the Host Controller within sixteen (16) ‘bit-times’ of said Host Controller issuing said instruction. In practise, this ensures that the USB Specification provides for a high efficiency of bandwidth utilization by limiting the period during which no information is being transmitted. However, these requirements also limit the physical range of USB devices . . . ” (Id., col. 2, lines 17-43.) The U.S. Pat. No. 6,381,666 describes a method for transmitting an isochronous USB data stream between a host controller and a peripheral device over an extended distance which includes the following steps: “a. feeding a first original, outgoing digital signal from a host controller to a local expander unit, and in response to determining that the local expander unit does not already possesses data requested by said outgoing digital signal, delivering a synthetic data packet having a payload of zero length to the host controller; b. transmitting said outgoing digital signal as an outgoing transmission signal, over a signal distribution system; c. receiving said outgoing transmission signal as a first original outgoing digital signal at a remote expander unit; d. delivering said first original outgoing digital signal from said remote expander to at least one peripheral device; e. receiving, at said remote expander, a reply digital signal from said at least one peripheral device; f. transmitting said reply digital signal as a reply transmission signal over said signal distribution system; g. receiving said reply transmission signal as a reply digital signal at said local expander; h. storing said reply digital signal as a stored reply digital signal until the receipt of a subsequent original, outgoing digital signal from said host controller, which subsequent signal is the same as, or similar to, said first original outgoing digital signal; and i. forwarding said stored reply digital signal to said host controller in response to said subsequent original outgoing digital signal.” (Id., claim 1 of the reexamined patent; see also FIG. 12, FIG. 9.) The synthetic data packet containing a zero length payload mentioned in step a. can be used to satisfy the timing requirements of the USB protocol while causing no disturbance to the actual information being carried by the protocol. (Id., col. 15, lines 58-63.) The U.S. Pat. No. 6,381,666 describes two other methods for transmitting an isochronous USB data stream; in these two methods, in response to determining that the local expander unit does not already possesses data requested by said outgoing digital signal, the local expander transmits no response, or transmits a negative acknowledgement packet (NaK) to the host controller (see Id., FIG. 8 and FIG. 19, respectively).